Method of and arrangement for processing through low temperature heat exchanges and in particular for treating natural gases and cracked gases

ABSTRACT

A method of processing through low temperature heat exchanges natural gases and cracked gases consisting in using a cooling cycle including compressing and then cooling a cycle mixture of gases consisting of C1 hydrocarbons, C2 hydrocarbons and C3 hydrocarbons through outer coolers with attendant total liquefaction of the cycle mixture under pressure; subsequently effecting a sub-cooling of the mixture within heat exchanges in counter-current relation to one expanded part thereof and/or cold returns and rejects of products processed in the cycle and finally expanding said sub-cooled mixture in at least one exchanger where various cooling processes of the treated products are carried out at successive temperature levels.

The present invention relates to a method of and an arrangement forprocessing through heat exchanges at low temperatures more particularlyranging between -30° C and -140° C, in particular for the purpose ofpurification or cleaning, fractional distillation and the treatment ingeneral of natural gases and cracked gases.

Through such refrigerating treatments it is in particular possible afterpassing through suitable exchangers to obtain a separation in separatorsof fractions of the gas it is desired to separate. It is thus forinstance known to "demethanize" a mixture of gases consistingessentially of hydrocarbons by separating the gaseous methane from theheavier hydrocarbons separated in the liquid state within a separatorthen referred to as a "demethanizer". Likewise it is possible toseparate within an other separator ethylene and ethane from the heavierhydrocarbons, in particular from C3+-containing hydrocarbons, such aseparator being then referred to as a "de-ethanizer". In such aseparator ethylene is extracted in a gaseous condition at the head ortop of the separator whereas ethane and the heavier hydrocarbons areseparated in a liquid phase from ethylene.

Such treatment operating steps are well known and very effective withrespect to separation and purity of the recovered products.Unfortunately the known ethylene-cycle refrigerating process usedrequires the use of complicated apparatus and the consumption of a largeamount of power inherent in the cycle used.

The object of the present invention is to provide improvements to thecooling cycle used with a view to simplifying the apparatus, decreasingthe power consumed for a same amount of treated products and thereforereducing the cost of the treatment.

The method of processing through heat exchangers at low temperaturesmore particularly ranging from -30° C to -140° C, in particular for thepurpose of purification, fractional distillation and the treatment ingeneral of natural gases, and cracked gases is characterized accordingto the invention in that it consists in making use of a cooling cyclecomprising the compression and then the cooling of a cycle mixture ofgases essentially comprising C1-containing, C2-containing andC3-containing hydrocarbons through outer coolants such for instance aswater, air and propane down to about -30° C, -40° C, said cooling beingattended by an at least partial liquefaction of said cycle mixture underpressure; then sub-cooling said mixture in heat exchangers incounter-current relation to one expanded part thereof and/or coldreturns and rejects of products treated in the cycle; and finallyexpanding said sub-cooled mixture within at least one exchanger wherethe various treatments are carried out for cooling the process productsat various successive temperature levels ranging from the lowest levelfor instance at about -100° C to -140° C to a less low level forinstance at about -50° C to -30° C. With such a method the thermalefficiency of the exchanges is improved thereby making it possible toreduce the power consumed for every treated unit of products.

The arrangement enabling to carry out the method according to theinvention is characterized in that it comprises :

a circuit of refrigerating fluid having several components consistingessentially of C1-containing, C2-containing and C3-containinghydrocarbons;

a station for compressing said fluid in the gaseous state from anabsolute pressure of about 1 to 2 bars at the suction or intake side ofthe compressors up to an absolute pressure of 30 to 50 bars at thedischarge or delievery side;

successive outer staged coolers working for instance with water, withair and with propane or propylene providing for the full condensationand/or the at least partial sub-cooling of said mixture aftercompression;

and at least one main exchanger where the expansion and vaporization ofsaid cycle mixture down to the pressure of return flow to thecompressors and the various heat exchanges for the cooling at thedesired different temperature levels of the processed products arecarried out.

Such an arrangement calls for a significantly simplified apparatus ofmore economical manufacture and maintenance.

The invention will appear more clearly as the following descriptionproceeds with reference to the accompanying drawings given by way ofexample only and wherein:

FIG. 1 shows a diagram for processing through refrigeration ofconventional type; and

FIG. 2 shows an improved diagram for treating through refrigerationaccording to the invention.

Reference should be made at first to FIG. 1 where there is illustratedan arrangement for treating cracked gases through staged coolings byliquefied ethylene and undergoing vaporization under three pressuresstages according to three successive temperature levels.

In the example shown the cooling circuit makes use of ethylene as acycle refrigerating gas. The gaseous ethylene issuing from separator 10under an absolute pressure of 1.65 bars is compressed successively inlow pressure stage 11, the medium pressure stage 12 and high pressurestage 13 of the compressors thereby bringing ethylene up to an absolutedelivery pressure of 32.4 bars. The ethylene is thus cooled in a firstwater cooler 14 and then in two successive coolers 15, 16 operating forinstance with propane, thereby making it possible to obtain liquidethylene at about -10° C under 32 bars in the condenser tank 17. Theethylene is then sub-cooled to a lower temperature in an exchanger 18working for instance with propylene or propane thereby bringing itstemperature down to about -30° C.

The ethylene is then expended from the pressure of 32 bars through anexpansion valve 19 down to the pressure of the separator 20 where it isof about 11.3 bars. This expansion provides the lowering of thetemperature of ethylene in a boiler-exchanger 21 which making itpossible to carry out a first refrigerating treatment at about -30° C,-45° C of products to be processed fed in at 22. These products may inparticular be cracked gases, produced ethylene and as a general rule anycharge it is desired to treat at such a temperature level. Ethylenewhich has been vaporized in the exchanger 21 is then carried through theduct 23 into the separator 20. The expansion valve 19 is operated by alevel controller 24 which provides for the permanent proper operation ofthe boiler-exchanger 21. In order to control the liquid ethylene levelat about -48° C in the separator 20 there is provided a by-pass duct 25in which is mounted an expansion valve 26 controlled by a controller 27for the liquid level in the separator 20.

The gaseous ethylene from the separator 20 flows back to the highpressure stage of the compressor 13, whereas the liquid ethylene isconveyed through the duct 28 towards an expansion valve 29 which feeds asecond boiler-exchanger 30 subjected to the pressure from separator 31,namely 5.2 bars in the examplary embodiment shown.

This expansion makes it possible to obtain in boiler 30 a temperaturelevel of the liquid ethylene of about -70° C thereby making it possibleto treat the products fed in at 33 at a temperature level of about -50°C to -67° C. At 34 has been shown a controller for the level of liquidethylene within the boiler 30 which automatically operates the expansionvalve 29.

The level of liquid ethylene at -70° C within the separator 31 iscontrolled by a level controller 35 which actuates a valve 36 insertedin a by-pass circuit 36 in parallel relation to the valve 29 and theboiler-exchanger 30.

The gaseous ethylene flowing out from the separator 31 is drawn under apressure of 5.2 bars into the inlet of the medium pressure stage 12 ofthe compressor, whereas the liquid ethylene at -70° C is conveyedthrough the duct 38 to the expansion valve 39 for feedingboiler-exchanger 40 at a lower temperature level of about -95° C in theexamplary embodiment shown. In this exchanger 40 are processed theproducts fed in at 41 at temperature levels of about -80° C to -92° C.At 42 is shown a level controller for liquid ethylene within theexchanger 40 which controls the valve 39.

The ethylene issuing in the gaseous condition from the exchanger 40 isrecovered or collected in the last separator 10 at a temperature of -95°C. The separator 10 performs the function of avoiding liquid beingpossibly carried along at the suction or input side of the compressor11. The possible residual liquids 43 are recovered or collected at thebottom of the separator 10 and may possibly be withdrawn from time totime through a drain valve or bleed cock 44.

In addition to the relative complexity of the apparatus used, thearrangement described hereinabove exhibits specific drawbacks such inparticular as:

A lack of flexibility in conducting the process operations, thetemperature levels at the three exchanger stages 21, 30 and 40 being setessentially by the pressures prevailing within the respective separators20, 31, 10, i.e. by the suction or input pressures at the threecompression stages 11, 12, 13. Since with a given compressor thecompression ratios are not variable, it is in practice not possible tomodulate the processing temperature at the three exchangers 21, 30, 40,which therefore are not used under optimum conditions of thermodynamicexchanges.

In the cycle dipicted in FIG. 1 it is found that the outside cooling ofthe ethylene cycle is effected within the exchangers 14, 15, 16, 18.Taking into account the temperature levels it is found that, apart fromthe exchanger 14, it is in practice necessary to operate the exchangers15, 16, 18 with a refrigerating fluid or coolant, for instance with apropane or propylene cycle, which accordingly should extract towards theoutside a major portion of the heat extracted by the ethylenerefrigerating cycle. This results in the requirement of providing alarge refrigerating circuit working with propylene and requiring the useof a large compressor for the propylene cycle.

The diagram of an improved arrangement designed according to theinvention is now described with reference to FIG. 2.

According to this diagram, there is provided a refrigerating circuitwhich does not make use of a pure substance such as ethylene but uses amixture of gases more particularly comprising C₁ -containing, C₂-containing and C₃ -containing hydrocarbons and possibly C₄ and N₂. Asuitable cycle mixture comprises for instance:

C₁ : 7% to 12%

C₂ : 30% to 45%

C₃ : 50% to 60%.

Good results have been achieved with a cycle mixture of gases having thefollowing composition:

C₁ : 8%;

c₂ : 37% to 42%;

C₃ : 50% to 55%.

More generally and according to particular utilization requirements thecycle mixture can have a composition selected among the following valueranges:

N₂ : 0% to 3%

C₁ : 5% to 25%

C₂ : 30% to 60%

C₃ : 30% to 60%.

Other compositions of the cycle mixture which are especially welladapted to the cracking of liquid batches or charges in particular ofnaphtha gasolines (whereas the compositions referred to hereinabovepreferably apply to the cracking of gaseous charges or batches) are thefollowing:

C₁ : 20% to 40%

C₂ : 30% to 60%

C₃ : 5% to 15%

C₄ : 5% to 15%.

A specific example is:

methane: 30%

ethylene: 30%; ethane: 20%

propane: 10%

isobutane: 10%

Also suitable for the cracking of liquid charges or batches are thefollowing compositions of cycle mixtures:

N₂ > 0.5%

c₁ : 20% to 40%

C₂ : 30% to 60%

C₃ : 20% to 35%.

A specific example is:

nitrogen: 1.5%

methane: 30%

ethylene: 20%; ethane: 20%

propane: 28.5%.

In the plant diagram shown this gaseous cycle mixture which has worked,i.e. extracted the heat required for the products processed within amain exchanger 50 is conveyed through a pipe-line 51 and a receivingstorage tank 52 in a superheated gaseous condition at -35° C under 1.5bars to the suction side of the first stage 53 of the compressors. Thepurpose of the receiving storage tank 52 is to avoid any possible liquidbeing carried along at the intake of the compressor 53 in case of atechnical hitch or possible defect in operating the plant. In theexamplary embodiment shown the cycle mixture is compressed within twosuccessive compression stages 53, 54 separated by an outer intercooling55 for instance by means of water.

The cycle mixture under a pressure of 34.5 bars which is the deliverypressure of the second compression stage 54 is then cooled within twosuccessive outer coolers 56, 57 for instance by means of water or air.When issuing from the cooler 57 more than one half of the cycle mixturehas already been condensed. The whole condensation of the cycle mixtureis continued within two exchangers operating for instance on propane orpropylene 58, 59 thereby making it possible to achieve the fullcondensation of the cycle mixture within the tank 60 at 10° C under 34bars. Within this tank the gaseous cycle mixture has been fullyliquefied.

The liquefied cycle mixture is then sub-cooled within two successiverefrigerating stages 61, 62 working for instance with propane orpropylene thereby bringing the temperature of the cycle mixture down to-32° C within the duct 63 before flowing through the main exchanger 50and secondary exchanger 64. Although these sub-cooling stages are notindispensable they usually promote a flexible and economical operatingcontrol of the process.

The exchanger 64 receives at 65, 66 the cold rejects of any kind fromthe plant and for instance cold rejects between -40° C and -80° C ofproducts treated within the main exchanger 50, for instance methanederived from a demethanizing step, etc. The exchanger 64 therefore makesit possible to recover one part of the energy expended to cool theserejects and to use the same for more thoroughly sub-cooling one part ofthe cycle mixture fed into the exchanger 64 through by-pass line 67. Thesub-cooling temperature may for instance reach -50° C at the outlet ofthe exchanger 64.

The remainder of the cycle mixture which does not flow through theby-pass pipe-line 67 is fed through pipe-line 68 into the exchanger 50in counter-current relationship with the cycle mixture entering the heador top of the exchanger 50 after expansion through the valve 69 whichprovides for the vaporization of the cycle mixture and hence the coolingof the exchanger 50. The fraction of the cycle mixture flowing throughthe pipe-line 68 is accordingly sub-cooled upon flowing through theexchanger 50 before entering the expansion valve 69.

At 70 is shown a valve for controlling the pressure of the sub-cooledcycle mixture having flown through the exchanger 64 and conveyed throughthe by-pass pipe-line 71 to the expansion valve 69. At 72, 73 are showndevices for controlling the pressure control valve 70 and the expansionvalve 69, respectively.

On account of the cycle mixture not being a pure substance, theexpansion of such a mixture within the valve 69 will result within theexchanger 50 in the gradual vaporization the lightest fractions of themixture being vaporized at first at the lowest temperatures and the lesslighter fractions then being vaporized at a less lower temperature.Thereby, there is obtained within the exchanger 50 a temperature stagingfrom the inlet for instance at about -100° C, to the outlet for instanceat about -40° C. The exchanger 50 advantageously is of plate-typeconstruction as well as the recovering exchanger 64.

The exchanger 50 lends itself easily to the positioning of variousseparated exchange circuits at differing temperature levels. In FIG. 2are shown six exchange circuits corresponding to six possibilities ofprocessing in parallel different products fed at 75, 76, 77, 78, 79 and80, respectively, into the exchanger 50. The circuits 75, 80 arelow-temperature circuits corresponding to some extent to the boiler 40shown in FIG. 1. The circuits 76, 79 are medium-temperature levelcircuits corresponding to some extent to the boiler 30 shown in FIG. 1.The circuits 77 and 78 are circuits working at less lower temperaturescorresponding somewhat to the boiler 21 illustrated in FIG. 1.

A comparison between both diagram of FIGS. 1 and 2 straightforwardlyshows that an arrangement according to the invention is very muchsimplified in terms of apparatus as compared with the plant shown inFIG. 1.

Moreover, the diagram according to FIG. 2 makes it possible to achievemore flexibility in controlling the operations and running the plantbecause the level temperatures of the staged circuits within theexchanger 50 will automatically adjust themselves to the requirementswhich is not the case with the exchangers 21, 30, 40 shown in thediagram of FIG. 1.

Also the diagram according to FIG. 2 behave thermodynamically in a muchmore favorable manner than a plant according to the diagram of FIG. 1,thereby making it possible to achieve substantial power savings forproviding the same refrigeration power.

On the other hand it appears from the diagram according to FIG. 2 that agreat part of the condensation of the cycle mixture is provided by theouter coolers 56, 57 which may operate for instance by means of air orwater, thereby making it possible to reduce to a large extent the powerand size of the propane or propylene refrigeration cycle which providesfor the full condensation and the first sub-cooling of the cyclemixture. In some cases it is possible to use one single air-operated orwater-operated refrigeration cycle only for providing the fullcondensation of the cycle mixture within the drum 60.

It is at least apparent that by modifying the composition of the cyclemixture it is possible to alter or vary without affecting the apparatusthe temperature and exchange conditions in particular at the exchanger50 and therefore to best adapt such a composition to the requirements ofthe plant.

The figures corresponding to the temperatures and pressures shown inFIGS. 1 and 2 are given by way of example only and to facilitatecomparisons, it being understood that with respect in particular to thediagram according to FIG. 2 the pressures and temperatures shown canvary to a rather substantial extent according to requirements. Thus anormal temperature range between the inlet and the outlet of theexchanger 50 would be between -140° C and -30° C.

More specifically, the suction during the compression step willgenerally be performed at an absolute pressure of between 1 bar and 3bars, whereas the absolute discharge or delivery pressure will rangefrom 30 to 50 bars. The temperature levels on the refrigerating fluidside of the exchanger 50 will usually range for the lowest level from-140° C to -100° C and for the less low level from -30° C to -50° C.

It should be understood that the invention is not at all limited to theform of embodiment and reduction to practice described which have beengiven by way of example only. The invention comprises all the technicalequivalents of the means described as well as their combinations if thelatter are carried out according to its gist and used within the scopeof the appended claims.

What is claimed is:
 1. In a method for refrigerating a natural gas or acracked gas by indirect heat exchange with a cycle gas mixture attemperatures ranging from -40° C. to -140° C., the improvementcomprising the sequence of stepscompressing a cycle gas mixtureconsisting essentially of C₁, C₂ and C₃ hydrocarbons, cooling saidcompressed cycle gas mixture in a plurality of successive stagedexternal, indirect heat exchanges with coolants selected from the groupconsisting of water, air, propane and propylene to about 40° C., wherebysaid cycle mixture is fully condensed, dividing the fully condensedcycle mixture into at least an auxiliary stream and a remaining stream,passing said auxiliary stream through an auxiliary heat exchangercountercurrently in indirect heat exchange with a colder stream employedin refrigerating the natural gas, whereby said auxiliary stream issub-cooled, passing said remaining stream through a main heat exchangercountercurrently in indirect heat exchange with a combined liquefiedsub-cooled cycle mixture as defined below after the latter has beenexpanded and passed into said main heat exchanger, whereby saidremaining stream is sub-cooled, combining said sub-cooled auxiliary andremaining streams to form said combined liquefied sub-cooled cyclemixture, expanding said combined sub-cooled stream and passing it at atemperature of from about -100° C. to about -140° C. countercurrentlyinto said main heat exchanger in indirect heat exchange with saidremaining stream and with a warmer stream employed in refrigerating thenatural gas.
 2. A method according to claim 1, comprising the step ofmaking said cycle mixture work between staged absolute pressures rangingfrom 1 bar to 3 bars at the suction side of the compressors and from 30bars to 50 bars at the delivery side of the compressors.
 3. A methodaccording to claim 1, comprising the step of using as a cycle mixture amixture comprising about 7% to 12% of C₁ hydrocarbon, 30% to 45% of C₂hydrocarbons and 50% to 60% of C₃ hydrocarbons.
 4. A method according toclaim 3, wherein the cycle mixture is a mixture comprising about 8% ofC₁ hydrocarbon, 37% to 42% of C₂ hydrocarbons and 50% to 55% of C₃hydrocarbons.
 5. A method according to claim 1, wherein the cyclemixture is a mixture comprising about:N₂ : 0% to 3% C₁ : 5% to 25% C₂ :30% to 60% C₃ : 30% to 60%.
 6. A method according to claim 1, whereinthe cycle mixture is a mixture comprising:C₁ : 20% to 40% C₂ : 30% to60% C₃ : 5% to 15% C₄ : 5% to 15%.
 7. A method according to claim 6,wherein said cycle mixture has the following composition:methane: 30%ethylene: 30%; ethane: 20% propane: 10% isobutane: 10%.
 8. A methodaccording to claim 1, wherein the cycle mixture is a mixture having thefollowing composition:N₂ > 0.5% c₁ : 20% to 40% C₂ : 30% to 60% C₃ : 20%to 35%.
 9. A method according to claim 8, wherein said cycle mixture hasthe following composition:nitrogen: 1.5% methane: 30% ethylene: 20%;ethane: 20% propane: 28.5%.
 10. A method according to claim 1, whereinsaid cycle mixture contains at least one of ethylene and propylene. 11.A system for refrigerating a natural gas or a cracked gas with a fluidcycle comprising multi-component refrigerant fluid at a low temperatureof from -40° C. to -140° C., the multi-component refrigerant fluidconsisting essentially of C₁, C₂ and C₃ hydrocarbons, the systemcomprising:compression means for compressing said refrigerant fluid inthe gaseous condition from an absolute pressure of about 1 bar to 3 barsat the suction side of the compression means to an absolute pressure ofabout 30 bars to 50 bars at the delivery side of the compression means;successive stage external coolers coupled to said compression means andoperating with coolants selected from the group consisting of water,air, propane and propylene for providing at least full condensation ofsaid refrigerant fluid after compression; at least one main heatexchanger coupled to at least one of said successive stage externalcooler for receiving therefrom a first part of said condensedrefrigerant fluid, and including inlet means and cold outlet means forsaid first part of said condensed refrigerant fluid, an expansion meansat its cold end at a temperature of from about -100° C. to about -140°C. for expansion of said refrigerant fluid to the pressure at thesuction side of the compression means, a vapor passage coupled to theexpansion means for passing vaporized refrigerant fluid through the mainexchanger for sub-cooling said first part of said condensed refrigerantfluid in indirect heat exchange therewith, and further inlet and furtheroutlet means containing a warmer stream than said refrigerant vapor inindirect heat exchange relation with vapor in said exchanger; at leastone auxiliary heat exchanger having an inlet means and a cold outletmeans, the inlet means thereof being coupled to said at least one ofsaid successive stage external coolers for receiving a second part ofsaid condensed refrigerant fluid, and having further inlet and furtheroutlet means containing a colder stream than said second part of saidcondensed refrigerant fluid for sub-cooling said second part of saidcondensed refrigerant fluid; means for coupling said vapor passage atits warm end to the suction side of said compression means; and aconduit connected to and joining the cold outlet means of said main andauxiliary heat exchangers for combining said sub-cooled first and secondparts of said condensed refrigerant fluid, and connected to saidexpansion means.
 12. A system according to claim 11, wherein at leastone portion of said cycle mixture is liquefied after having flownthrough said outer coolers operating with at least one of water and air.13. A system according to claim 11, wherein said main exchanger is usedfor sub-cooling at least one portion of said cycle mixture incounter-current relationship with itself prior to expansion andvaporization within said exchanger.
 14. A system according to claim 11,further comprising a secondary exchanger for carrying out through heatexchange with cold process rejects issuing from said main exchanger thesub-cooling of at least one part of said cycle mixture before expansionand vaporization within said main exchanger.
 15. A system according toclaim 11, wherein said exchangers are of plate-type construction.
 16. Asystem according to claim 15, wherein said main exchanger is divided onthe refrigerated side into several independent components for theseparate treatment of various products at given temperature levels.